The present invention relates to a power-assisted steering system for wheeled vehicles, and more particularly to an improvement of a changeover valve unit for the power-assisted steering system which includes a spool valve element for controlling the supply of fluid to a hydraulic cylinder associated with the steering system.
In a conventional changeover valve unit for such a power-assisted steering system as the rack-and-pinion type, a cylindrical spool valve element is concentrically assembled with an input shaft which is coaxially connected to an output shaft for relative rotation caused by the operator's effort applied to the steering wheel. The spool valve element is axially displaced upwards or downwards in response to relative rotation of the input and output shafts to change over the flow of pressurized fluid between a hydraulic cylinder associated with the steering system and a source of fluid pressure in the form of a fluid pump. To provide a displacement mechanism for the spool valve element, the input shaft is formed at the outer periphery of its inner end portion with helical follower grooves in which steel balls are rotatably contained and engaged with a portion of the spool valve element in such a manner that the spool valve element is displaced proportionally to the relative rotation angle between the input and output shafts. In this displacement mechanism, it is required that the steel balls be rotatably retained by a retainer ring or collar with a slight clearance within the follower grooves to ensure smooth movement of the steel balls along the follower grooves. For this reason, an intervening space inevitably occurs between the steel balls and the follower grooves. This results in increase of hysteresis in the fluid control characteristic of the changeover valve unit, as shown by imaginary lines in FIG. 1. This means that a smooth return operation of the steering wheel may not be expected.
It has also been experienced that axial vibration of the spool valve element occurs due to fluctuation of the fluid pressure caused by the pulsation of the fluid pump and that the operator encounters unpleasant noises caused by the axial vibration of the spool valve element. For the purpose of eliminating such a defect, a compression coil spring is interposed between one end of the spool valve element and a bearing for the input shaft to resiliently urge the steel balls against one side-wall of each of the follower grooves so as to restrain the vibration of the spool valve element. However, if the load of the compression spring is insufficient, the desired effect may not be expected, and if the load of the compression spring becomes excessive upon displacement of the spool valve element, the operational feel of the steering wheel will suffer.